Collimators are used in many applications in order to define the shape and alignment of radiation (which may be electromagnetic waves or beams of particles). For example it is possible to create two-dimensional fan-shaped beams of radiation or one-dimensional pencil beams of radiation using collimators. In particular applications of collimation, such as those using electromagnetic radiation in the visible or near visible spectrum, mirrors and lenses can be used to produce collimated beams. However for electromagnetic radiation with significantly shorter wavelengths and therefore higher energy (such as X-rays and Gamma-rays) or for radiation in the form of beams of particles, a collimator that acts as a filter to the radiation is required, such that only radiation travelling in desired directions is able to pass through the collimator unhindered.
Collimation is a necessity in many areas of physics and medicine where it is desirable to confine a divergent source of radiation into a useful, well-defined beam. Use of collimated beams of radiation enable a number of different analysis techniques to be performed and leads to improved resolution in some imaging applications, by minimising the amount of radiation that interacts with material that is not under test. Example applications where collimated beams of radiation may be required include X-ray and Gamma-ray radiography, radiation therapy and neutron imaging. Collimators may also be used to filter radiation from a scene, such that only radiation from a specific direction is allowed to pass through to, for instance, a detector. Further example applications where the ability to detect radiation from specific directions may be useful are Gamma-ray observations of space, and in the analysis of radioactive material.
Typically, a collimator for high energy electromagnetic radiation is made from a material of high atomic number such as tungsten or lead, and defines a number of apertures through which radiation can travel towards a target or detector. Radiation that is incident upon the body of the collimator is attenuated, so that only rays aligned with the apertures pass through unhindered.
A common problem with collimation techniques is that the flux at the target is greatly reduced as most of the source waves are blocked by the body of the collimator. This hinders imaging and analysis techniques by reducing performance and image clarity or by increasing the power of the source needed to attain the same image clarity at equal penetration. Furthermore, inconsistency in collimation effect (for instance with different beam angles) can further complicate imaging and analysis techniques.
Certain imaging applications such as x-ray backscatter, require the use of a scanning beam of radiation to build up a two-dimensional image of an object or field of view. A scanning beam can be achieved by introducing relative movement between the radiation source and the collimator in one dimension to produce a strip of image. If such one-dimensional scanning is combined with relative movement between the object and the source in an orthogonal direction, multiple one-dimensional strip images can be combined to form a two-dimensional image. It is known that to create a scanning pencil beam, a radiation source can be placed at the centre of a collimator in the form of a large rotating disc provided with radial apertures. As the disc rotates, a beam is emitted through each aperture and scanned across the field of view. However, such a disc is necessarily large and heavy. This affects the weight and portability of the whole equipment, requires significant power to maintain the correct rate of rotation and requires multiple moving parts, all of which increase the risk of equipment failure through breakage.
An alternative collimator design, disclosed in U.S.2014/0010351 (Rommel), utilises two parallel plates separated by a distance d. Each plate comprises a slot with the slots being arranged in a crossed arrangement to form an “X” or “+” shape. For radiation approaching from a given angle there is only a single compound aperture which passes through both slots, however as relative movement between the source and the collimator is introduced in one dimension, the single compound aperture “moves” in a lateral dimension. Therefore, by moving either the source or the collimator up and down, a laterally scanning beam can be created.
In the parallel plate collimator example, the path length through the compound aperture varies with displacement along the length of the apertures. This leads to a variation in the collimation effect and a variation in the size and shape of the beam exiting from the collimator, both of which have a negative impact on the quality of the final image. This latter problem is addressed in U.S.2014/0010351 (Rommel) by manipulating the shape of the slots. By increasing the width of the slots towards the edges of the block it is possible to maintain a constant beam cross-section area independent of the beam angle. However, the variance in path lengths remains, affecting the quality of collimation.
A further design of collimator is the solid cuboid twisted slit collimator. Such a collimator is illustrated in EP2124231 (BAM). For this collimator the path length through the compound aperture varies with displacement along the length of the slit, thereby resulting in variable collimation effect. Furthermore, in applications where a scanning beam of radiation is required, the solid cuboid twisted slit collimator needs to be rotated back-and-forth, rather than spun continuously, thus limiting achievable scanning speeds.
Therefore it is an aim of the invention to provide a collimator for providing constant collimation effect over a plurality of beam angles, combined with simplicity of design.